Some 74,000 years ago, a volcano called Toba on the Indonesian island of Sumatra unleashed one of the greatest eruptions ever known, spreading ash across southern Asia. The catastrophe had witnesses. Archaeologists digging beneath the ash layer have found stone artefacts indicating that humans were living in the Jurreru Valley of southern India before the eruption. But were they modern humans, or some other, now extinct group?
     When modern humans left Africa for Asia - as well as the routes they followed, the tools they carried, and the reasons they went - are all controversial.
     The first question is, pre-Toba, or post-Toba? Paul Mellars at the University of Cambridge argues that modern humans left Africa long after the Toba eruption, 60,000 years ago at the earliest. Equipped with new technologies, including bows and arrows, they beach-hopped along the coastline of the Arabian peninsula, India and southeast Asia, reaching Australia in short order.
     Michael Petraglia at the University of Oxford, UK, on the other hand, is convinced that people spread into Asia at least 74,000 years ago, and perhaps as early as 125,000 years ago - well before Toba, during a wet, warm interlude between ice ages - carrying tools no more sophisticated than those made by earlier humans, and wandering along river valleys and lake shores.
     Researchers have mostly relied on the DNA of living people to reconstruct the ancient story - focusing on isolated native groups thought to be descendants of early human settlers in their areas, collecting mitochondrial DNA (mtDNA - inherited only from the mother), identifying distinctive variants (haplotypes), comparing them, and using estimates of mutation rates as a molecular clock.
     The origin is marked by a haplotype called L3 that appeared before humans left Africa, an estimated 60,000 to 70,000 years ago - suggesting humanity left Africa a few thousand years after Toba. The three next-oldest haplotypes - immediate descendants of L3 outside Africa - are 60,000 to 65,000 years old. All three gave rise to multiple bursts of genetic variation, from Arabia to Bali. "For that to happen," says geneticist Stephen Oppenheimer of the University of Oxford, "people would have had to move very fast, before new mutations occurred."
     The most plausible route for a rapid migration is along the coast of the Indian Ocean. By the mid-2000s, most researchers accepted the 'coastal express', post-Toba migration scenario.
     Stephen Oppenheimer studies mtDNA and contributed to the coastal-express model. In his view, the migration had to have been rapid and coastal - but not necessarily as recent as others have insisted. DNA is not a precise clock. A clue to an earlier date, he says, comes from mtDNA studies in India. One set of variants was less diverse than expected, suggesting to him that the first modern humans in India suffered some kind of catastrophe that reduced their numbers to almost nothing. The cause, he speculated, might have been the Toba ash cloud.
     Petraglia and his supporters say that evidence for the earlier migration has grown much stronger in the past two years. A group led by Jeffrey Rose of the University of Birmingham, UK, found 106,000-year-old stone relics in Oman, consisting of distinctive triangular cores and the long spear points made from them - technology first excavated at sites inhabited by modern humans in Nubia, in northern Sudan, more than 2,000 kilometres away and on the other side of the Red Sea. "There's no question," says Rose. "It's the same people."
     At the time, the Arabian peninsula was a moist savannah teeming with game. When the climate cooled and dried about 75,000 years ago, turning Arabia back into a desert, the Nubian pioneers either died out or retreated to Africa, but other ancient Arabian populations might have wandered farther into Asia.
     At a site in the United Arab Emirates called Jebel Faya, Simon Armitage of Royal Holloway, University of London, and his colleagues have found even older artefacts, dating to as early as 125,000 years ago and resembling objects made by modern humans in eastern Africa. "It's extremely plausible that a population at Faya could have moved on," Armitage says.

Edited from Nature Magazine (2 May 2012)